1. Field of the Invention
The present invention is related to a method and apparatus for mounting a plurality of circuit elements such as integrated circuits and memory elements on a printed wiring board. In particular, the present invention is directed to a terminal density conversion board and connection structures for integrated circuits mounted on the printed wiring board.
2. Description of the Related Art
In recent years, high-speed, low-cost multi-chip modules (MCM) having a plurality of integrated circuit parts mounted on high-density printed wiring boards have become the subject of industrial attention. These multi-chip modules are classified roughly into three types: MCM-L, MCM-C, and MCM-D. The MCM-L type mounts circuit elements on a printed wiring board allowing reductions in production costs. The MCM-C type mounts circuit elements on a thick-film multi-layer ceramic board achieving a moderately high speed at a low cost. The MCM-D type module has circuit elements installed on a circuit board having laminated dielectric and conductive wiring layers on a thick-film substrate such as a ceramic board.
These multi-chip modules are mounted on printed wiring boards (also referred to as circuit boards) together with other circuit elements, which may include active elements such as LSI chips, resistors, and capacitors.
Mounted parts, such as the multi-chip modules and the other circuit elements, have many terminals. The pitches or distances between the terminals of these mounted parts are fixed values. The terminals of these mounted parts are connected to the terminals formed on a printed wiring board. The terminals on the printed wiring board are connected with the multi-layer wiring inside the printed wiring board.
The speed and performance of circuit elements, such as multi-chip modules, mounted on printed wiring boards can be rapidly increased, as compared with other mounted parts, resulting in a radical miniaturization and proliferation of terminals. Naturally, reducing the size of the interterminal pitch on such circuit elements has thus become necessary. Correspondingly, for printed wiring boards on which such mounted parts are mounted, the reduction of the interterminal pitch length is deemed to be inevitable. That is, the interlayer connecting vias formed on a baseboard must be very dense (i.e., tightly packed together) to be used for printed wiring boards for such increasingly faster and improved multi-chip modules.
The printed wiring board is generally a large baseboard, because many circuit elements are mounted thereon. It is extremely difficult to install the highly dense vias over the total area of the baseboard uniformly and flawlessly. Therefore, the yield for this kind of printed wiring board is lower than that of boards forming lower density vias, and this increases the production cost of the overall structure including the mounting parts (such as multi-chip modules) and printed wiring boards on which these mounting parts are mounted.
It is an object of the present invention to solve the above-described problems in the art.
It is another object of the present invention to obviate the need to increase the via density of the printed wiring board in order to accommodate increasingly faster and improved integrated circuit parts with a higher-density terminal pitch.
It is yet another object of the present invention to improve the product yield and reduce production costs for printed circuit boards mounted with many circuit parts.
It is a further object of the present invention to reduce the cost of materials, increase product yield, and otherwise facilitate the proper miniaturization and improvement of integrated circuit parts having higher-density terminal pitches.
It is also an object of the present invention to reduce the costs for and improve the degree of freedom in design of a single baseboard configuration having a large number of different types of mounted integrated circuits and complex printed wiring distributions.
It is another object of the present invention to ease repairs, re-patterning, replacement, upgrading, and troubleshooting of the printed wiring board and the integrated circuits mounted thereon.
It is a further object of the present invention to more effectively utilize free space on a density conversion board to improve mountability of electrical parts onto the printed wiring board.
It is also an object of the present invention to reduce thermal stress between the integrated circuit and the printed wiring board.
Objects of the present invention are achieved by providing a method and apparatus for mounting circuit elements on a printed wiring board, wherein an integrated circuit having terminals with a first interterminal pitch are mounted onto a first surface of a terminal density conversion board which converts the first interterminal pitch of the integrated circuit to terminals with a second interterminal pitch larger than the first interterminal pitch on a second surface of the terminal density conversion board; and the terminals on the second surface of the terminal density conversion board with the second interterminal pitch are mounted onto the printed wiring board.
Objects of the present invention are also achieved by mounting circuit elements having terminals with the second interterminal pitch onto the printed wiring board at locations other than the location at which the terminal density conversion board is mounted onto the printed wiring board. Alternatively, circuit elements having terminals with the second interterminal pitch may also be mounted onto the terminal density conversion board at locations other than the location at which the integrated circuit is mounted onto the terminal density conversion board and at locations other than the location of the terminals on the terminal density conversion board connecting the terminal density conversion board to the printed wiring board.
Objects of the present invention are further achieved by using different connections between the terminal density conversion board and the printed wiring board, including a soldered metal, pin joints, Z-shaped contacts on which conductive pads on the terminal density conversion board are press-fitted, and a land grid array securing the terminal density conversion board on the printed wiring board.
Objects of the present invention are also achieved by providing a cooling device mounted onto a surface of the integrated circuit (i.e., the surface without the terminals connected to the terminal density conversion board), and setting a thermal expansion coefficient of the terminal density conversion board to a value between a thermal expansion coefficient of the integrated circuit and a thermal expansion coefficient of the printed wiring board.